A proportional pressure control valve is known in which a spool is moved to a position corresponding to a magnetic force for changeover of connection of a load port to a high pressure source port and a low pressure source port and for adjustment of degree of opening of variable throttles for communication of the load port with the high pressure source port and the low pressure source port, thereby controlling the pressure at the load port (hereinafter referred to as "load pressure"), e.g. from unexamined Japanese patent publication 1-261581 or unexamined Japanese utility model publication 7-6579.
In the control valve of the former publication, the spool is biased from both sides by springs to keep a neutral position, and the load pressure is applied to one end of the spool, and a thrust produced by a proportional solenoid is applied to its other end. Also, in order to case a plunger (actuating element) of the proportional solenoid to follow the spool, it is pressed against the other end of the spool by a spring.
On the other hand, in the fluid pressure control valve of the latter publication, in a normal state, an output port is in communication with a drain port. When a plunger (actuating element) of a pressure-increasing solenoid is attracted by an electromagnet, the spool connected to the plunger moves in such a direction that the output fluid pressure increases. Also, a reaction force piston for applying a returning force to the spool using the output fluid pressure is inserted in one end of the spool on the side where the pressure-increasing solenoid is provided, and further the plunger of the pressure-increasing solenoid is supported by a bush for guiding its movement.
The control valve of the former publication needs three springs, so that the structure is complicated and the cost is high due to increase in the number of parts.
Also, in the fluid pressure control valve of the latter publication, since the reaction force piston inserted in the spool is provided on the same side as the plunger (actuating element) of the pressure-increasing solenoid fitted on the spool, it is necessary to form a hole (oil path) in the plunger at the portion where the magnetic flux density of the plunger portion becomes high so that no pressure difference is produced between both ends of the plunger. Thus, the volume of the portion in question decreases. This results in the loss of power of the electromagnet.
Also, in either control valve, because the plunger of the soleniod is supported by a bush which serves as a slide guide, the slide resistance of the plunger adds to the slide resistance of the spool, thus affecting the movement of the spool unfavorably.
Further, in either control valve, because the reaction force produced by the reaction force piston (referred to as "slide pin" in the former publication 1-2615819) is applied to the spool in such a direction against the spool driving force from the solenoid, extra power is needed for the electromagnet, so that the power consumption increases.
An object of this invention is to solve these problems.